| The fixture is probably not grounded properly, or at all. Check to be sure, if you have Romex NM-B cable, that the bare wire is grounded, and connected to the frame of the fixture using a machine thread screw. A new fixture is supplied with a green screw just for this purpose.
If using BX (AC) cable, be sure the sheathing is grounded, and the cable is connected to the fixture using the proper cable connector.
If there is no grounding wire grounding the fixture,(equipment ground) either a bare, green wire or the metal sheathing of a cable or conduit assembly, the fixture will always having problems operating properly, no matter how many parts are changed.
What Don is referring to is voltage drop caused by a heavy amp load on the circuit, that would cause the voltage at the fixture to drop dramatically.
I doubt this is the case, but it is possible.
There will need to be a large reefer, or AC unit, or a high wattage electric heater, or something along those lines, that comes on everytime the light does not worked properly. Or a loose connection in the circuit causes a voltage drop, this is somewhat likely.
If you notice the light works well on damp humid days/nights, and not so well on dry low humidity days, grounding is the cause.
Re-reading your post, you state that turning the switch off and back on again helps bring the light on again.
I stand by my defective ground advice. I have seen that symptom many times.
Yeah, I have handled a couple fixtures in my time, a few hundred thousand I would guess. That comes from installing lighting systems and working lighting maintenance for years.
If you need more detail to troubleshoot and/or repair, get back with more detail of the wiring and fixture. Pictures tell a thousand words. | 
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